Positive maternal affect during mother–litter interaction is reduced in new mother rats exhibiting a depression-like phenotype

The experience of positive affect during new motherhood is considered essential for a healthy mother–infant relationship, with life-long consequences for both mother and child. Affective availability and contingent responsiveness are often compromised in mothers experiencing postpartum depression, yet how maternal affect impacts parenting is not fully understood. In this study, we used the Wistar-Kyoto (WKY) rat model of depression and ultrasonic vocalizations to examine the relationship between maternal affect and parenting. We examined the affective and behavioral response of WKY and control new mother rats during social interactions with their offspring. Our results show that WKY mothers displayed altered USV signaling accompanying substantial disturbances in their maternal caregiving. In addition, WKY mothers failed to adjust vocal frequency in coordination with offspring proximity and interaction compared to control mothers. A follow up experiment demonstrated that the administration of the adenosine A2A receptor antagonist MSX-3 ameliorated both maternal behavioral deficits and low positive affect in WKY mothers. Together, our results highlight the importance of maternal positive affect in the dyad relationship and suggest a role for the striatopallidal pathway in the affective processing of parenting.

The experience of positive affect during early new motherhood is considered fundamental for the healthy relationship between the mother and her infant, and the wellbeing of each [1][2][3] . Postpartum depression and other maternal neuropsychiatric disorders are characterized by low positive affect and attenuated feelings of pleasure and comfort with the infant, which has negative implications for the mother-infant relationship and the infant's developmental outcome [4][5][6][7][8][9][10] . Mothers suffering postpartum depression are often less sensitive to their child's needs and signals, vocalize less often, are affectively flat and less engaged during positive social interactions with their infants [11][12][13][14][15] . Despite the strong association between maternal affect and parenting, little is understood about how the affective experience of motherhood impacts parenting.
In this study, we examined the relationship between maternal affect and parenting, including the impact of depressive-like symptomatology on mothers' behavior and affective expression during interactions with their SD and WKY mothers vocalize more when with their offspring. Immediately after R1: Mother Alone, the litter was returned to the testing room and placed next to the maternal cage, so that the mother was able to see, smell, and hear her pups, but not physically interact with them, and a second 5-min recording of the mother was taken (R2: Mother with Litter Separated). As shown in Fig. 2b, with the return of the offspring to the testing room, both SD and WKY mothers significantly increased the number of calls (Strain, F (1,14) = 0.095 p = 0.76 η 2 p = 0.007; Context, F (1,14) = 6.3 p = 0.025 η 2 p = 0.31; Strain × Context interaction, F (1,14) = 0.5 p = 0.49 η 2 p = 0.035), primarily due to an increased expression of 50 kHz calls (Strain, F (1,14) = 0.18 p = 0.68 η 2 p = 0.012; Context, F (1,14) = 5.9 p = 0.029 η 2 p = 0.29; Strain × Context interaction, F (1,14) = 0.55 p = 0.47 η 2 p = 0.038), although there was significant variability among WKY mothers. Specifically, only 4 of 9 WKY mothers vocalized more when with their offspring, whereas all 7 SD mothers did so (7/7 vs 4/9, Fisher's Exact Test p = 0.034) (Fig. 2b). The expression of 22 kHz calls decreased, although it didn't reach statistical significance (Strain, F (1,14) (1,14) = 0.00 p = 0.99 η 2 p < 0.001), with both SD and WKY mothers similarly emitting significantly more flats and shorts in the presence of their young.
SD mothers, but not WKY mothers, vocalize at a higher frequency when with their offspring. To examine whether mothers modify the acoustic properties of their USVs when in the presence of the offspring, we compared the peak frequency and duration of relevant call categories between recordings. SD mothers increased the peak frequency of their calls, including flats and shorts when with their offspring (Flats, F (1,100)   Representative spectrograms of USV call categories emitted by SD and WKY mothers and their PND7-8 pups (frequency is plotted along the y-axis and duration along the x-axis). (a) Multi-step and trill 50 kHz call categories and 22 kHz calls were only observed during mother-only recordings. (b) Flat, short and most FM calls were observed during both mother-only (R1 and R2) and offspring-only (R4, R5 and R6) recordings. Flat: A USV with near-constant frequency and a mean slope between − 0.2 and 0.2 kHz/ ms. Short: A USV with a duration of less than 12 ms. Complex: A USV that contains two or more directional changes in frequency of at least 5 kHz each. Inverted-u: A USV with a steady increase followed by a steady decrease in frequency greater than 5 kHz each.
Step down: A USV with an instantaneous frequency jump greater than 10 kHz to a lower frequency.
Step up: A USV with an instantaneous frequency jump greater than 10 kHz to a higher frequency. Downward ramp: A USV with a steady decrease in frequency with a mean slope less than − 0.2 kHz/ms. Upward ramp: A USV with a steady increase in frequency with a mean slope greater than 0.2 kHz/ms. Trill: A USV with rapid oscillations in frequency. Multi-step: A USV with more than one instantaneous frequency jump greater than 10 kHz. . Immediately after the maternal behavior test, the mother and litter were removed from the home cage and placed in a new clean cage outside the testing room. For the isolation-induced recordings, one pup was placed back in the home cage outside of the nest quadrant and was recorded for 5 min. The pup was then returned to the mother, and a pup of the opposite sex was then placed alone in the home cage. After both a male and a female pup were recorded, the mother and pups were returned to the home cage. Figure 3a-d shows the number and acoustic characteristics of all call categories emitted by SD and WKY pups. Representative spectrograms for each of the call categories emitted by pups are displayed in Fig. 1, including FM, short and flat 50 kHz calls.
SD and WKY pups have a similar vocal reaction to isolation. In response to separation from their mother and littermates, both WKY and SD pups emitted a similar profile of USVs (χ 2 = 5.5, df = 6, p = 0.48), mostly flat and   Fig. 3c). Also, the latency to start calling (Strain, F (1,26)  WKY pups emit calls at a higher frequency. Analysis of the acoustic properties of the isolation-induced USVs revealed that WKY pups emitted isolation-induced calls at a higher peak frequency than SD pups (Flat Peak Frequency: Strain, F (1,26) Fig. 3a). Immediately after R4/5 isolation recordings, body weights and core temperatures were collected, as these variables are known to alter pup USV emission 46,48,65 . Pups were reunited with their mother immediately after examination. As expected, there was a significant difference in body weight between strains, but not between sexes, with WKY pups being significantly lighter than SD pups (SD female 19.20 ± 0.29 g, SD male 19.34 ± 0.45 g, WKY female 11.99 ± 0.37 g, and WKY male 12.08 ± 0.53 g. Strain, F (1,26) = 304.9 p < 0.001 η 2 p = 0.92; Sex, F (1,26) = 0.071   Fig. 3b), while no significant correlations between body weight or body temperature with the number or duration of these isolation-induced USVs were detected.
Social context impacts SD and WKY pups' rate of USV emission. To further evaluate the offspring vocal repertoire, specifically whether offspring USVs change with social context, we recorded offspring's USVs in the presence of their mother and littermates (R6: Litter with Mother Anesthetized). Both mother and litter were removed from the maternal cage, the mother was anesthetized and immediately returned to the nest quadrant of the home cage. The litter was housed in a small cage until testing. Fifteen min later, an initial 5 min recording of the anesthetized mother verified the absence of maternal USVs. The pups were then scattered in the home cage away from the nest quadrant, and their USVs were recorded for 15 min.
As during R4/5 isolation recordings, SD and WKY pups emitted flat, short, and a variety of frequently modulated USVs, but no trills (Fig. 3g). To discern the effects of different social contexts on pup USV emission, the number, duration and frequency of the USVs were compared between minutes 1-3, 7-9, and 13-15 of the test. During the first 1-3 min, the pups were scattered throughout three quadrants of the cage without direct skin contact with their mother or littermates. By minutes 7-9, the pups had grouped together within their quadrant. Ten minutes into the recording, the litter was manually grouped around the anesthetized mother in the nest quadrant by the experimenter. Therefore, in minutes 13-15 of the recording, the pups were grouped with their anesthetized mother in the nest quadrant. The number of USVs similarly declined with increased social contact for both strains, but SD litters emitted significantly more USVs, especially flats, during the first 3 min of the recording compared to WKY litters (Flats: Strain, F (1,13)  Notably, once grouped with their mother and siblings, both SD and WKY pups emitted very few vocalizations (Fig. 3e).
WKY mothers experience low positive affect during interactions with their offspring. Acoustic analysis of USVs in mother-only (R1 and R2) and offspring-only recordings (R4, R5 and R6) revealed differences between mothers and offspring in the mean duration and frequency of calls. Specifically, mothers emitted calls that were longer than those emitted by their offspring (e.g., Flat Duration: SD MOTHER 0.0817 ± 0.0129 s vs SD PUP 0.0568 ± 0.0036 s, t(12) = 1.844 p = 0.045; WKY MOTHER 0.0771 ± 0.0094 vs WKY PUP 0.0580 ± 0.0032 s, t(16) = 1.801 p = 0.045. Short Duration: SD MOTHER 0.00812 ± 0.00109 s vs SD PUP 0.00572 ± 0.00029 s, t(12) = 2.118 p = 0.028; WKY MOM 0.00775 ± 0.00078 s vs WKY PUP 0.00551 ± 0.00032 s, t(16) = 3.212 p = 0.003). In addition, mothers emitted flats at a higher frequency than their offspring (Flat Frequency: SD MOTHER 52.79 ± 3.97 kHz vs SD PUP 41.23 ± 1.29, t(12) = 2.780 p = 0.008; WKY MOTHER 56.16 ± 2.99 vs WKY PUP 48.89 ± 2.39, t(16) = 1.897 p = 0.038). Accordingly, upper bound cutoff durations (and frequency) of each USV call category emitted during the offspring-only recordings were used, within each dyad, to identify those USVs emitted by the mother during the 30-min social interaction with her offspring (e.g., within each dyad, flat calls in the R3 mother-litter recording with durations above R4-6 values were assigned to mothers). Similarly, upper bound cutoff frequencies were used to detect maternal flats. Lastly, trills and multi-steps were not observed during any offspring-only recordings, and thus calls within these USV categories were assigned to mothers (see Fig. 1). While we are aware our strategy likely didn't account for all maternal calls, we are confident we identified the majority of the calls (> 85%) emitted by mothers (i.e., acoustic parameters, trills, minimal overlapping, etc. See below) during interactions with their offspring.
When only considering maternal USVs, a significant difference in the number of USVs emitted by SD and WKY mothers was found, with WKY mothers emitting significantly less calls than SD mothers during interactions with their offspring (t(17) = 2.878 p = 0.006; Fig. 4f,g). The USV profile also differed between strains, with WKY mothers emitting a smaller proportion of flats and a higher proportion of trills than SD mothers, particularly during retrievals and active caregiving (χ 2 = 59.4, df = 3, p < 0.01 and χ 2 = 90.5, df = 3, p < 0.01, respectively; Fig. 4h). In addition, a significant positive correlation was found between the maternal behavior performance and the number of flats (r = 0.577, p = 0.019). In contrast, a negative correlation was found between the maternal behavior performance and the number of maternal trills (r = − 0.713, p = 0.002) (Fig. 4f).
To determine the relationship between maternal USVs and caregiving behaviors, we analyzed the video recordings synchronized with the audio recordings. Maternal USVs during the following behavioral categories were examined: (i) retrievals (mother retrieves pups to the nest; (ii) active caregiving (mother is in the nest hovering over the pups, while performing active caregiving behaviors); (iii) nursing (mother adopts a quiescent nursing posture over the pups) and iv) away from pups (mother engages in non-maternal activities outside the nest). As expected, most calls during retrievals were emitted by pups. Most calls emitted thereafter were emitted by mothers during active caregiving, when all pups were grouped in the nest and emitted very few vocalizations (see R6 results above). This is further supported by the low proportion of overlapping calls once the litter was grouped in the nest (Behavior, F (3,45)  . SD mothers emitted a lower rate of USVs during retrievals but a higher rate of USVs during active caregiving than WKY mothers (p = 0.018 and p = 0.000, respectively) (Fig. 4g). SD mothers were responsible for most flats and shorts emitted during active caregiving, whereas less than 30% of calls belonged to SD mothers during retrievals (Flats, retrievals vs active caregiving p = 0.000; Shorts, retrievals vs active caregiving p = 0.000). In contrast, WKY mothers similarly emitted low rates of flats and shorts during interactions with pups (retrievals vs active caregiving both p = ns), which was www.nature.com/scientificreports/ significantly different compared to SD mothers (Flats, retrievals p = 0.048 and active caregiving p = 0.000; Shorts retrievals p = 0.94 and active caregiving p = 0.000). Of note, the higher rate of maternal USVs in WKY mothers during retrievals was mostly due to their higher emission of trills. SD and WKY mothers similarly emitted very few calls while away from their young or during nursing (p = 0.41 and p = 0.25, respectively; Fig. 4g).
In addition, SD mothers synchronized USV expression to moments of social interaction with their young, especially during active caregiving, whereas WKY mothers did not. Thus, SD mothers prevalently emitted most of their USV when with their young (vs away performing non-maternal active behaviors p < 0.001 and p = 0.001, respectively), whereas WKY mothers emitted similar USV rates across all behavioral categories (active caregiving vs away, p = 0.678; active caregiving vs nursing p = 0.480; nursing versus away, p = 0.12).
Of note, the number of USVs emitted, and the USV profiles, differed markedly when comparing the two mother-litter recordings (R3 vs R6). In the first 15 min of recording with an awake behaving mother (R3), dyads emitted approximately three times the average number of calls than dyads with an anesthetized mother (R6) (main effect of recording: F (1,13) = 15.133, p = 0.002 η 2 p = 0.538), indicating a significant maternal contribution to USVs. Moreover, the number of identified maternal USVs accounts for the difference between recordings.

Experiment 2: Effect of the adenosine A 2A receptor antagonist MSX-3 on maternal behavior and USV emissions of WKY mothers.
We then examined the ability of the adenosine A 2A receptor antagonist MSX-3 to ameliorate the behavioral deficits and low positive affect of WKY mothers. Separate groups of SD and WKY postpartum female rats were randomly assigned to receive IP injections of either saline vehicle (VEH) or 1.0 mg/kg of MSX-3 20 min prior to maternal behavior testing. A dose of 1.0 mg/kg of MSX-3 was chosen based on our published findings and preliminary data showing that it effectively ameliorated maternal behavior deficits in both SD mothers treated with D2 dopamine (DA) receptor antagonists and WKY mothers 66 .

MSX-3 ameliorates deficits in maternal behavior in WKY mothers.
Consistent with our prior studies, the maternal behavior of vehicle-treated WKY mothers was minimal and disorganized compared to SD mothers ( Fig. 5a-d).  Fig. 5a-d). MSX-3-treated WKY mothers more readily grouped all their pups into the nest (Latency to group p = 0.01), spent significantly more time with their pups (Time in contact p = 0.003), and licked and groomed them more (Corporal Licking: p < 0.001; Anogenital Licking: p < 0.001) than vehicle-treated WKY mothers, with expression levels comparable to those displayed by SD mothers (e.g., SD VEH vs WKY MSX-3 : Retrieval p = 0.89; Latency to group p = 0.161; Duration hover over p = 0.116). In addition, a significant majority of MSX-3-treated WKY mothers grouped all pups into the nest, licked and nursed their litters during the test compared to vehicle-treated WKY mothers (45% WKY VEH vs 83% WKY MSX-3 , 100% SD VEH/MSX-3 vs 83% WKY MSX-3 , Fisher's Exact Tests p = 0.09 and p = 0.481, respectively; Fig. 5a). In contrast, MSX-3 had no effect on the retrieval quality of WKY mothers, with most WKY mothers, regardless of treatment, retrieving pups by body parts other than the typical nape of the neck compared to SD mothers' typical neck retrievals (% of neck retrievals: Strain, F (1,38) Fig. 5f-i). Collapsed across WKY groups, there was a significant negative correlation between the number of trills and maternal performance (r = − 0.706, p = 0.02), indicating once more the inverse relationship between maternal performance and trill emission (Fig. 5f). MSX-3 also significantly altered the call profile of WKY mothers, but not that of SD mothers, primarily by reducing the proportion of trills and increasing the proportion of flats (WKY VEH vs WKY MSX-3 χ 2 = 18.8, df = 4, p = 0.0008; SD VEH vs SD MSX-3 : χ 2 = 1.7, df = 4, p = 0.79; Fig. 5i).
MSX-3 had no effect on any acoustic parameter of calls emitted by SD and WKY dyads (e.g., Flat Frequency: Strain, F ( Fig. 5h).

MSX-3 increases USV rate during social interaction in WKY mothers.
Because MSX-3 did not affect the acoustic properties of the USVs emitted by SD and WKY dyads, strain-specific mean cutoff values determined in Experiment 1 were used to identify the USVs emitted by SD and WKY mothers in Experiment 2. Examination of USVs emitted by WKY mother during maternal caregiving categories revealed a significant effect of MSX-3 on the USV rate (Treatment, F (1,14) = 4.41 p = 0.054 η 2 p = 0.24; Behavior, F (3,42) = 142.8 p < 0.001 η 2 p = 0.9; Treatment × Behavior, F (3,42) = 6.4 p = 0.23 η 2 p = 0.31), with MSX-3 reducing the emission of trills during retrieval (p = 0.03) and increasing the emission of calls, mostly flats during active caregiving (p = 0.04) compared to the vehicle-treated WKY group (Fig. 5g,h). In contrast, MSX-3 had no effect on the USV expression of SD mothers in any behavioral category (Treatment, F (1,13) = 0.2 p = 0.66 η 2 p = 0.015; Behavior, F (3,39) = 346.1 p < 0.001 η 2 p = 0.96; Treatment × Behavior, F (3,39) = 0.26 p = 0.65 η 2 p = 0.02) (Fig. 5g,h). Of note, compared to results from Experiment 1, Experiment 2 showed overall less USVs, regardless of strain and treatment, suggesting that the injection itself had an impact on USV expression (Fig. 5e). Notably, this reduction in USVs was mostly observed during active caregiving behavioral category, and not during retrievals, indicating that the mothers' USVs, and not the offspring's USVs, were affected by IP injection. This finding is www.nature.com/scientificreports/ consistent with previous reports that rats receiving IP injections call less 67 , and further supports the accuracy of our assignment of USVs to mothers. No differences were found between Experiments 1 and 2 for any maternal behaviors (i.e., SD vs SD VEH and WKY vs WKY VEH , all ps = ns).

Discussion
The present study shows that mother rats robustly emit 50 kHz USV calls during social interaction with their offspring, and that the number and profile of maternal USV calls predicts their maternal interest and caregiving efforts. Specifically, control SD mother rats emitted high rates of 50 kHz USVs, mostly flats and shorts, during contingent interactions with their offspring, which have been suggested to indicate positive affect. In addition, SD mothers adjusted the way they vocalize when with their offspring, by increasing the frequency and duration of their vocalizations. In contrast, WKY mothers exhibiting a depressive phenotype displayed substantial disturbances in maternal caregiving, fewer calls and an altered USV profile. Additionally, WKY mothers did not synchronize their USVs to moments of social interaction, nor did they change their vocal frequency when with their offspring, further indicative of disrupted interactions with their young. Administration of the adenosine A 2A receptor antagonist MSX-3 ameliorated both maternal behavioral deficits and low positive affect in WKY mothers. These results highlight the varying degree of positive affect that is experienced with new motherhood and the importance of maternal positive affect in the dyad relationship. Consistent with prior work, our results find that WKY mothers exhibit severe deficits in their maternal behavior compared to control strains 57,63,68,69 . WKY females responded to being reunited with their pups following separation with minimal active caregiving, spending most of the test time away from their young. When interacting with their young, WKY mothers were less engaged and disorganized, and handled their pups more roughly, often dropping and/or stepping on pups, and retrieving pups by the head, limbs, side, or belly, instead of by the typical nape of the neck. This is in high contrast to SD mothers, who directed most of their behavior toward their pups, with little display of non-maternal activities such as self-grooming, resting and feeding during the test time. Our prior work has demonstrated that these caregiving differences between SD and WKY mothers are not driven by (i) differences in their locomotor activity, as both SD and WKY mothers are equally active in a 30-min test, (ii) by the handling associated with testing, or (iii) related to developmental or sensory characteristics of the pups, as they are also evident during undisrupted observations in the home cage and in cross-fostering experiments 63,70 . In this sense, clinical studies indicate that postpartum depression is significantly associated with less maternal responsiveness, and hostile and disengaged parenting 9,71,72 .
Both SD and WKY mothers exhibited a positive affective response during social interactions with their offspring, as indicated by 50 kHz USVs. Further supporting the positive experience of interaction was the emission of 22 kHz USVs, which are indicative of experiencing a negative emotional state 22 , uniquely during periods of separation from their young by both SD and WKY mothers. Consistent with published studies, following reunion with their offspring, both SD and WKY mothers emitted only 50 kHz calls and at a higher rate 55,56 , although there were substantial differences in their affective reaction. SD mother rats emitted high rates of 50 kHz calls, mostly flats and shorts, during contingent caregiving interactions with their young. In contrast, WKY mothers emitted significantly fewer calls and in a different profile, including fewer flats and more trills, and this altered USV profile was significantly correlated with their disrupted maternal performance. Significantly, trills were almost exclusively emitted by WKY mothers and mostly during exposure and interaction with scattered pups (i.e., during R2 and beginning of R3), indicating that WKY mothers exhibit a different emotional reaction to offspring's calls than control SD mothers. In support, the number of trills emitted by WKY mothers predicted the severity of their parenting disturbances, with those WKY mothers emitting the highest trill rates, failing to group their pups in the nest and spending most of the test time away from them. Flat USVs are highly expressed during social interaction and have been suggested to be involved in social contact, coordination and reward 27,41 . Trill USVs, on the other hand, are commonly emitted during high emotional arousal in both appetitive and aversive situations 27,29,31,34,41,[73][74][75][76][77] . Noteworthy, FM USVs that occur in aversive contexts are sensitive to anxiolytics and antidepressants [76][77][78][79] , further supporting their interpretation as signaling an anxiety-like state. Taken together, our results suggest that WKY mothers might find the demands of caregiving overwhelming and experience low positive affect during social interactions with their offspring, which strongly correlates with their rough and fragmented maternal behaviors. In support, baby cries produce amplified feelings of anxiety and panic in mothers with depression, who are more likely to perceive their child as difficult and to respond with irritability when their children make normal demands of care 9,80-83 .
Significantly, SD mothers, but not WKYs, shifted to a higher vocal frequency when with their offspring. In addition to this vocal shift, SD mothers synchronize their USVs to moments of social interactions with their offspring, further suggesting that these maternal calls are indicative of the mother's positive affect. In contrast, WKY mothers lacked coordination of vocalizations with social interacting moments. This result is consistent with clinical studies showing that mothers experiencing postpartum depression speak less to their infants and are less likely to utilize offspring-directed vocalization ('motherese' or 'baby talk') during interactions with their infants 84,85 . In humans and other mammals, the use of 'motherese' is thought to indicate affect and to promote social interactions that are critical for the affective, cognitive, and social development of the offspring 84,[86][87][88][89][90] .
The reduced caregiving and positive affect of WKY mothers was accompanied by their offspring's altered USVs, potentially reflecting disrupted affiliative behavior of WKY pups related to the ongoing insensitive interaction style of their mothers. Although SD and WKY pups emitted similar call rates and profiles throughout most of the social conditions examined, WKY pups showed altered calling patterns when scattered in the home cage away from the nest and their mother. In agreement, one previous report showed reduced maternal potentiation of USVs and proximity-seeking behaviors in WKY pups 91 . Likewise, several reports have shown that rat pups alter their vocalizations when exposed to infrequent and/or rough maternal interactions 43 www.nature.com/scientificreports/ studies have shown a similar pattern, with infants of mothers suffering from depression being more likely to display an insecure attachment to their mothers than infants of control mothers [93][94][95] . It is also likely that offspring behavior in turn influences the mother's maternal responsiveness and affective state, promoting a cycle of dysfunctional interactions. Unfortunately, limited studies have examined infant influences on their mother's affective well-being 96 . Future studies should consider the dynamic and reciprocal nature of the mother-infant relationship to better understand the bidirectional influences on the dyad's affective wellbeing. In addition, WKY pups' higher peak frequency of calls were consistent across all recordings, suggesting inherent differences between strains. Consistent with our prior study, WKY pups gain weight, reach developmental milestones, and thermoregulate similarly to SD pups 63 , arguing against a developmental delay. One likely explanation, however, is related to their size difference, as WKY pups are ~ 7 g smaller than SD pups. In support, and consistent with a previous study 97 , a significant negative correlation was found between body weight and the frequency of the USVs emitted by rat pups. In addition, acoustic parameters of USVs, including peak frequency, change as the pups grow and develop 43,45,[98][99][100] .
Otherwise, SD and WKY pups had similar call rates and profiles throughout the different social contexts examined. These results are highly consistent with previous work highlighting isolation-induced vocalizations in rat pups and the contact quieting response following reunion with littermates and mother, as well as the increase in USVs rates during retrieval, regardless of whether retrievals are performed by the mother or by an experimenter 43,52,101 . Our finding that both SD and WKY pups similarly emitted little calling while in the nest also confirms prior reports 102,103 .
Administration of the selective A 2A receptor antagonist MSX-3 substantially ameliorated the active caregiving deficits of WKY mothers, to levels characteristic of SD mothers. Thus, MSX-3-treated WKY mothers readily approached their pups and spent most of the test time with their young, actively taking care of them. This result is consistent with previous findings demonstrating a reversal effect of MSX-3 in motivational deficits induced by DA antagonism, including haloperidol-induced deficits in maternal behavior 66,[104][105][106] . Notably, WKY's caregiving deficits resemble those of SD mothers following systemic and intra-accumbens administration of a DA receptor antagonist 66,[107][108][109][110][111] , indicative of the magnitude of the caregiving deficits in WKY mothers. Adenosine A 2A receptors are almost exclusively expressed in the striatum and highly colocalized with DA D2 receptors on GABAergic striatopallidal neurons, where they antagonize DA D2 receptor activity [112][113][114][115][116][117] . Consistent with prior work, administration of MSX-3 reversed the effects of haloperidol, but was without effect when administered alone, on the maternal behavior of SD mothers, suggesting that A 2A antagonists are effective in conditions of reduced DAergic activity but not under 'normal' conditions 66,118,119 . Taken together, these results suggest that blunted mesolimbic DAergic responsiveness to offspring underlies aspects of the deficits in active caregiving of WKY mothers. In agreement, WKY mothers have lower intracellular levels of all monoamines, including dopamine, as well as different patterns of change in their monoamine pathways as they transition across postpartum, compared to SD mothers 63 .
MSX-3 treatment also reduced the emission of trills and synchronized maternal USVs to moments of interaction with the offspring (i.e., increased the rate of calls, mostly of flats and shorts, during active caregiving) in WKY mothers, indicative of improved positive affect. Moreover, the reduced expression of trills correlated with improved maternal performance in WKY mothers, suggesting that the low positive affect and reduced motivational aspects of caregiving in WKY mothers are related to increased striatopallidal activity. This result is consistent with prior work demonstrating that activation of A 2A receptors with CGS21680 disrupts behavioral activation functions of motivated behavior and attenuates the emission of pro-social 50 kHz USVs in rats 120 . Similarly, the production of USVs in appetitive contexts is strongly related to the activity of VTA → NA DA neurons 121 . Of note, MSX-3 did not impact the USV expression or behavior of SD mothers, consistent with previous findings 119,120 . Taken together, reduced striatopallidal activity with MSX-3 ameliorated both maternal affect and caregiving in WKY mother, suggesting overlapping neurobiology. In support, effort-related motivational symptoms, well-recognized, highly debilitating aspects of depression, are highly correlated to problems with low positive affect and social function, and treatment strategies that target these symptoms have shown to be beneficial to positive affect and emotional wellbeing 118,[122][123][124][125][126] . Moreover, positive affect has been associated with increased working memory, cognitive flexibility and effort-related functions [127][128][129] , all processes key to parenting, suggesting a bidirectional benefit between maternal affect and parenting.
In conclusion, this study recapitulates the observed variations in positive affect experienced with new motherhood in humans, and suggests a common neurobiological substrate underlying maternal affect and contingent responsiveness toward offspring. Understanding how the maternal brain manifests positive affect and the neurobiological mechanisms by which maternal affect impacts parenting is not only essential to our understanding of how mothers mother, but also for developing more effective intervention strategies for depression and other postpartum neuropsychiatric disorders aimed at restoring the mother-infant relationship.

Materials and methods
Animals. Primiparous postpartum Sprague-Dawley (SD) and Wistar-Kyoto (WKY) female rats purchased from Charles River Laboratories (Kingston, NY) and approximately 90 days of age were used. Both SD and WKY strains are derived from the Wistar strain, and thus SDs have consistently been used as a control strain for the WKY strain 58,60-63 . Animals were maintained on a 12/12 h light/dark cycle (lights on from 7:00 A.M. to 7:00 P.M.) at 22 ± 1 °C with ad libitum access to food, water and sunflower seeds. Experienced SD and WKY male rats, ranging from 90 to 180 days of age, were used for mating in our laboratory. Before giving birth, pregnant females were housed in individual clear Plexiglass cages (38.5 cm × 48.5 cm × 20.5 cm) lined with fresh Sani-Chips ® bedding and containing Eco-Bedding nest-building material. On postpartum day (PPD) 1 (birth = day 0) litters were culled to 8 pups (3-5 males, 3-5 females) per mother rat. All experimental procedures followed General procedure. All behavioral procedures were conducted during the light phase of the light/dark cycle. One day before testing, a 5-cm high Plexiglas divider was inserted into each female's cage to divide the floor of the cage into four equal compartments. Dyads were tested in their home cage, which was placed into an adjoining testing room 15 min prior to starting the test. Room temperature was maintained at 22 ± 1 °C. A microphone was positioned above the cage, and a video camera recorded the behavior of the mother and/or pups for later offline analysis. Once the day's recordings were completed, females and their pups were returned to the colony room. Experiment 1: Maternal affect during social interaction with offspring. This experiment examined the affective responses of SD and WKY mother rats during social interaction with their offspring. In fast-paced, dynamic social interactions, it can be difficult to readily attribute calls to a specific member of the mother-infant dyad. In order to determine maternal vocalizations, PPD7-8 WKY and SD mothers and their pups underwent a series of recordings. Day 1 of testing began with the litter removed from the home cage and housed in a small cage (lined with bedding and containing nest-building material from the maternal cage) outside of the testing room. Ten minutes after the removal of the litter, a 5-min recording of the mother alone in her cage was taken (R1: Mother Alone). The cage containing the litter was then returned to the testing room and placed adjacent to the maternal home cage, so that the mother was able to see, smell, and hear her pups, but not physically interact with them, and a second 5-min recording of the mother was taken (R2: Mother with Litter Separated). Thereafter, the litter was scattered in the home cage opposite to the nest, and a 30-min recording was taken in conjunction with a maternal behavior test (R3: Mother-Litter Interaction). Immediately after, the mother and her litter were removed from the home cage. One pup from the litter was randomly selected and returned to the home cage, and a 5-min recording was taken from this pup in isolation (R4: Male/Female Pup Isolated). The pup was then reunited with his/her mother, and a littermate of the opposite sex was returned to the home cage for a 5-min recording (R5: Female/ Male Pup Isolated). Individual pups were placed outside of the nest site during isolation recordings. The order of the sex of the pups recorded was counterbalanced within groups. The pups' temperatures were measured during the isolation recording with an infrared temperature gun digital thermometer.
Day 2 of testing began with the mother injected intraperitoneally (IP) with 1.0 ml/kg of a solution that contained ketamine HCl (75.0 mg/mL), xylazine (7.5 mg/mL) and acepromazine maleate (1.5 mg/mL) before being placed into the testing room. Once the mother exhibited loss of palpebral/corneal and pedal withdrawal reflexes (~ 5 min), the litter was removed from the home cage and housed outside of the testing room. Fifteen minutes later, a 5-min recording of the anesthetized mother alone was taken to confirm the absence of maternal vocalizations. The litter was then returned and scattered in the home cage opposite to the nest, and a 15-min recording was taken (R6: Litter with Mother Anesthetized). During this recording, after 10 min had elapsed, the litter was grouped in the nest with the mother by a researcher, to compare litter vocalization levels before and after grouping. Experiment 2: Effect of the adenosine A 2A receptor antagonist MSX-3 on maternal behavior and affect of WKY mothers. This experiment examined the ability of the adenosine A 2A receptor antagonist MSX-3 to ameliorate the behavioral and vocal deficits of WKY mothers. Separate groups of SD and WKY postpartum female rats were randomly assigned to receive IP injections of either 1.0 mg/kg/ml of MSX-3 (Sigma Chemical, St. Louis, MO, USA) or same volume of corresponding vehicle. MSX-3 was freshly dissolved in 0.9% saline, which was also used as the vehicle condition. Twenty minutes before the maternal behavior test, both mother and litter were removed from the home cage, the mother received an injection of either MSX-3 or vehicle and was immediately returned to her home cage. The litter was housed in a small cage, lined with bedding and containing nest-building material from the maternal cage, outside of the testing room until testing. MSX-3 dose and injection time (1.0 mg/kg IP; 20 min before testing) were selected based on our previously published report showing that this dose effectively ameliorated haloperidol-induced maternal behavior deficits in SD mothers 66 . Maternal behavior test. Following 20 min of maternal separation, the entire litter was scattered in the home cage opposite to the nest, and the number, duration and latency of maternal behaviors were recorded continuously for 30 min, as previously described 63,66,108 . Other behaviors recorded included general exploration (line crosses and rearings), self-grooming and eating/drinking.

Ultrasonic vocalization (USV) recording and analysis.
USVs were recorded using a CM16/CMPA condenser ultrasound microphone connected via an Avisoft UltrasoundGate 116H acquisition device to a computer with Avisoft RECORDER software (sampling rate: 250 kHz; 16 bits; Avisoft Bioacoustics, Berlin, Germany). Acoustic analysis of the recorded .wav files was performed post hoc using Avisoft SASLab Pro software. Spectrograms were generated with a fast Fourier transformation (FFT) length of 512 points and a time window overlap of 75% (FlatTop window, 100% frame size). Correspondingly, spectrograms had a frequency resolution of 488 Hz and a temporal resolution of 0.512 ms. Analyses of USVs were performed blind with respect to experimental conditions by an experienced coder according to USV categories previously described 64 . A second coder independently analyzed a random subset of spectrograms and established inter-rater reliability higher than 90%. www.nature.com/scientificreports/ Acoustic features of each call, including duration, peak frequency and peak amplitude were measured by the automatic parameter measurement tool of the software.

Statistics.
A total of 41,622 USVs were manually detected and analyzed in this study. Based on visual inspection of the spectrograms, calls were categorized into 22 kHz or one of 10 50 kHz call categories 64 . In addition, USVs (< 1%) that did not fit any of the 12 categories were classified as "unclear". Most frequency modulated (FM) categories, except for trills, had a low proportion of emission (less than 2%) by all groups and experiments and were combined into the FM category for statistical analysis. The following main call categories were used for final analysis: (1) negative 22 kHz call, and 50 kHz (2) flats, (3) shorts, (4) trills and (5) other FM calls. Acoustic data are expressed as mean ± standard error of the mean (SEM). Calling rates, proportional production, and acoustic parameters (frequency, duration) of these USV categories were analyzed using linear mixed models, with strain (SD and WKY), recording (R1-R6) and/or treatment (vehicle and MSX-3) as factors for relevant comparisons. Significant main effects and interactions were further analyzed using Tukey's HSD tests. The χ 2 goodness-of-fit test was used to analyze USV profiles from R1 and R2 recordings of mothers. Behavioral data are expressed as mean ± SEM and were analyzed with independent-samples t test (Exp. 1) or two-way ANOVAs (Exp. 2) with strain (SD and WKY) and treatment (vehicle and MSX-3) as factors for relevant comparisons. Analyses of pups' body weight and temperature were performed with two-way ANOVAs with strain (SD and WKY) and sex (female and male) as the between-subjects factors. Between-group categorical data comparisons were examined using χ 2 test of independence and Fisher's exact tests. Pearson's tests were used for correlation analysis between number, duration or frequency of USVs and body weights, body temperature or maternal behavior. All analyses were performed using SPSS software (SPSS v25; IBM Corp., USA). Statistical significance was set at p < 0.05.

Data availability
The datasets generated and/or analyzed during the current study are available from the corresponding author on reasonable request.